Tire repair operations, such as tire retreading operations, are generally used to extend the useful service life of a tire. Typical tire retreading operations include removing previously worn tread from a tire and bonding new tread in its place. Tires may be retread or repaired one or more times as a less expensive alternative to purchasing new tires, providing particular advantages for large-scale operations such as trucking, bussing and commercial aviation.
Generally, some level of non-destructive testing (NDT) of the tire prior to repair is conducted to determine whether it is appropriate to perform the repair operation. Visual inspection methods can be used to validate the integrity and, subsequently, the viability of retread and/or repair of tire casings for retread. For instance, the inside and outside surface of a tire can be visually inspected by an operator using special lighting to inspect for defects such as crazing, cracks, snags, bulges, depressions, gouges, abrasions, marbling, bubbles, blisters, separations, and other defects. Visual inspection methods, however, are subjective, inconsistent, and can require extensive training. Moreover, due to high operator turnover, difficulty exists in retaining expertise.
High voltage discharge (HVD) testing can be performed in place of or supplemental to visual inspection. HVD testing can be used to identify anomalies in the inner liner of a tire that penetrate the insulating material of the inner liner. In HVD testing machines, the tread portion of a tire is typically disposed between a pair of electrodes across which a high voltage electrical potential is generated. The voltage applied across the electrode will cause electrical discharge at the location of a defect in a tire. U.S. Pat. No. 6,050,136, which is incorporated herein by reference for all purposes, for instance, discloses a HVD test machine that employs electrical discharging to detect defects in the inner liner of a tire.
On a traditional HVD test machine, the probe assembly typically includes a series of wire loops and small chains that are positioned to hang inside the tire in a manner to distribute high voltage from bead to bead on the inside surface of the tire. The correct width probe must be chosen for the tire size. The ground path for the discharge at an anomaly is provided by contact of the tread on a metallic driven roller. When the probe passes over an anomaly, an electrical discharge passes through the tread at the location of the anomaly to the metallic driven roller.
Traditional HVD test machines suffer from several disadvantages. For instance, traditional HVD test machines typically require manual selection of probe size to accommodate varying tire sizes. For instance, three different probe sizes may be provided to cover the range of retread capable truck tires. Once a probe size has been selected, the probe must be mounted semi-manually into the inner surface of the tire, causing the HVD testing machine to be susceptible to improper positioning.
In addition, because typical HVD probes cover the entire inside surface of the tire from bead to bead, when an anomaly is detected, it is unknown at what precise radial position the anomaly is located. Typically, the tire will stop rotating when a discharge is detected. This provides for an azimuth location of the anomaly. However, to obtain a precise radial location of the anomaly, the operator typically has to press and hold a manual button to repeat the discharge in order to mark the tire with a carbon deposit or to visually locate a corona discharge.
Furthermore, the detection capability of typical HVD test machines depends on many variables. For instance, the bend of the wires, the condition of the chains, the thickness of the tread, the speed of rotation, and the chemical makeup of the tread influence the detection capability of HVD test machines. Significant variability can occur with slight elevations changes of the tire surface, degradation or improper trimming of the chains, or degradation or improper positioning of the wires. For example, a slight elevation change in the inner surface of the tire may cause the HVD probe to temporarily leave the surface of the inner liner, causing the HVD probe to miss an anomaly in the tire surface.
Moreover, due to the cyclic charge and discharge nature of high voltage power sources used to energize HVD probes, detection of an anomaly is dependent on the probe being in close proximity to the anomaly when the high voltage charge is at a voltage level sufficient to discharge through an anomaly. The configuration of the chains and wires of the probe in relation to the tire dictates how much surface area of the probe is in contact with the tire. The tire surface must be rotated at a speed that is slow enough to ensure that the probe is sufficiently charged when the surface area of the probe is in contact with a given point on the tire surface to detect the presence of anomalies.
Thus, a solution is needed for automated HVD testing of tires that overcomes the above mentioned disadvantages. The solution can reduce the need for operator interaction to determine the accurate and precise location of tire surface anomalies. A high voltage probe that is less susceptible to variables, such as elevation changes in the surface of the tire and improper positioning of the high voltage probe, would be particularly useful. A high voltage probe that can be used with increased tire rotation speeds and that ensures contact with a given point on the surface of the tire when the high voltage probe is charged to a voltage level sufficient to discharge through an anomaly would also be particularly useful.